1. Technical Field
The present invention relates to a wire-bonding method of connecting a first bonding point with a second bonding point by a wire, and to a semiconductor device having a wire-loop shape formed by connecting a first bonding point with a second bonding point by a wire.
2. Description of the Related Art
For assembling a semiconductor device, wire bonding for connecting a pad of a semiconductor chip mounted on a lead frame with a lead of the lead frame by a thin metal wire is used. A wire-bonding apparatus is used in the wire bonding, in which an initial ball is first formed at a tip end of the wire, and the initial ball is pressure-bonded to the pad of the semiconductor chip using the capillary, thereby forming a pressure-bonded ball. In this method, after moving the capillary upward to make a reverse motion in a direction away from a second bonding point, the capillary is further moved upward to a predetermined height, and then moved toward the second bonding point, thereby the wire is connected to the second bonding point (see FIG. 4 to FIG. 6 of Japanese Unexamined Patent Application Publication No. 2004-172477 (hereinafter referred to as “Patent Document 1”), for example).
The wire is bonded by moving the capillary in this manner, and, in many cases, a loop shape of the wire is formed either in a triangular shape including a wire neck that extends upward from the pressure-bonded ball that has been pressure-bonded onto the pad of the semiconductor chip and a sloped portion that has been bent toward the second bonding point at the wire neck, or in a trapezoidal shape including a flat portion that extends substantially horizontally in a direction toward the second bonding point from the wire neck and a sloped portion that extends from the flat portion toward the second bonding point. This is because, when a portion close to the pressure-bonded ball is moved horizontally in the direction toward the second bonding point with respect to the capillary, the neck portion is often damaged due to a friction between the capillary and the thin metal wire that is produced while moving.
However, the loop height of the wire in this wire-loop shape is high because the wire neck that rises from the pressure-bonded ball is included, and this poses a problem that the height or thickness of the semiconductor device assembled by wire bonding as a whole cannot be made small.
Therefore, a method has been proposed, the method making a reverse motion such that the capillary is moved slightly upward and away from the second bonding point after bonding to a first bonding point, making a forward motion such that the capillary is further moved slightly upward and in the direction toward the second bonding point, then moving the capillary downward to press the wire neck portion against the pressure-bonded ball and fold the wire neck portion up on the pressure-bonded ball, making the wire to extend either in a horizontal direction or in a direction slightly sloped upward from the horizontal direction, moving the capillary upward while feeding the wire from a tip end of the capillary, and then moving the capillary toward the second bonding point, thereby connecting the wire to the second bonding point (see FIG. 1 to FIG. 3 of Patent Document 1, or FIG. 1 to FIG. 3 of Japanese Unexamined Patent Application Publication No. H09-51011 (hereinafter referred to as “Patent Document 2”), for example).
Also, another bonding method of bonding a first bonding point with a second bonding point has been proposed, the method forming a pressure-bonded ball at a first bonding point, moving the capillary slightly upward and then toward a second bonding point, moving the capillary downward by a distance shorter than a rising distance that the capillary has moved upward to press down the wire, and then moving the capillary upward and toward the second bonding point while feeding out the wire, thereby connecting the wire to the second bonding point (see FIG. 1 and FIG. 2 of Japanese Unexamined Patent Application Publication No. 2005-39192 (hereinafter referred to as “Patent Document 3”), for example).
Moreover, still another bonding method of forming a low-profile loop with an arch height of its wire loop of about 100 μm has been proposed, the method forming a pressure-bonded ball by bonding to a first bonding point, carrying out a first upward movement, a first downward and parallel movement, a second upward movement, and a second downward and parallel movement of the capillary, and finally bonding to the second bonding point. In this method, it is proposed that the capillary be moved up to a position that is several times higher than a desired arch height of the wire loop in the first upward movement and further moved by a length of the wire loop in the second upward movement, and the first and the second downward and parallel movements be carried out in an arc-wise manner (see FIG. 3 of Japanese Unexamined Patent Application Publication No. H08-316260 (hereinafter referred to as “Patent Document 4”), for example).
In addition, a further bonding method of bonding a first bonding point with a second bonding point has been proposed, the method bonding at a first bonding point to form a pressure-bonded ball and a ball neck, moving the capillary obliquely upward toward a second bonding point to make the ball neck sloped, carrying out a flexing step for a plurality of times of flexing the wire by the upward and parallel movement of the capillary, then moving the capillary upward and looping the capillary to the second bonding point, thereby bonding the wire to the second bonding point (see FIG. 1 to FIG. 4 of Japanese Patent No. 4137061 (hereinafter referred to as “Patent Document 5”), for example).
The conventional bonding method described in the Patent Document 1 or 2 poses a problem that the wire near the first bonding point can be damaged since the wire is folded up onto the pressure-bonded ball to form a head portion. Also, since the head portion cannot be made particularly low, it is occasionally not possible to satisfy the demand for decreasing the height of the wire loop as a whole.
In this regard, the conventional bonding method described in the Patent Document 3 does not form the head portion which is formed by folding up the wire on the pressure-bonded ball as in the case of the method described in the Patent Document 1 or 2, and the wire that is connected to the pressure-bonded is bent toward the second bonding point thereby connecting the wire to the second bonding point, and therefore it is possible to reduce the height of the wire loop as a whole as compared to the method described in the Patent Document 1 or 2. Moreover, in the conventional bonding method described in the Patent Document 5, it is possible to reduce the height of the wire loop while suppressing the damage to the wire near the first bonding point.
However, according to the conventional bonding method described in the Patent Document 3, a portion of the wire connected to the pressure-bonded ball is pressed toward the second bonding point in the horizontal direction by the capillary, and then the capillary is moved downward by the distance shorter than the rising distance of the capillary. As a result, the portion of the wire connected to the pressure-bonded ball is occasionally pulled along the axial direction of the wire due to the frictional force produced between the capillary and the wire when the capillary is moved horizontally. When the wire is pulled in this manner, the cross-sectional area of the portion of the wire connected to the pressure-bonded ball disadvantageously becomes small and the strength of the wire decreases. This could result in a problem of disconnection of the wire. Similarly, as the capillary is horizontally moved toward the second bonding point in the conventional bonding method described in the Patent Document 5, the portion of the wire connected to the pressure-bonded ball is pulled due to the frictional force produced between the capillary and the wire, and thus the wire strength adversely decreases.
In addition, according to the conventional bonding method described in the Patent Document 4, the capillary that has been moved upward is moved downward and parallelly along an arc-wise trajectory. Therefore, it is possible to prevent the decrease in the strength of the wire as in the conventional techniques described in the Patent Document 3 and Patent Document 5. However, a problem has been noted that the height of the wire loop can be only as low as about 100 μm, and it is adversely difficult to form a lower-profile loop.